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Asynchronous event handlers for served and shutdown #380

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Sep 4, 2023
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Asynchronous event handlers for served and shutdown #380

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27 changes: 27 additions & 0 deletions node.js/cds-server.md
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Expand Up @@ -179,6 +179,7 @@ cds.on('served', (services)=>{
})
```

This event supports _asynchronous_ event handlers.


### listening {.event}
Expand All @@ -191,10 +192,36 @@ A one-time event, emitted when the server has been started and is listening to i

A one-time event, emitted when the server is closed and/or the process finishes. Listeners can execute cleanup tasks.

This event supports _asynchronous_ event handlers.


### Event Handlers
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Could this be a level two header? And could both headers be one line? Like: Sync vs. Async Event Handlers or Event Handler: Sync/Async


#### Synchronous vs. asynchronous

Unless otherwise noted, event handlers execute **synchronously** in the order they are registered.
This is due to `cds.on()` and `cds.emit()` using Node's [EventEmitter](https://nodejs.org/api/events.html#asynchronous-vs-synchronous) contract.

In other words this asynchronous handler code does **not work** as expected:

```js
cds.on ('bootstrap', async ()=> {
await asyncCode() // [!code error] // will NOT be awaited
}
```

You can use the [served](#served) event's asynchronous nature though to wait for such bootstrap code:

```js
let done
cds.on('bootstrap', ()=> {
done = asyncCode()
}
cds.on('served', async ()=> {
await moreCode()
await done
})
```


## See Also...
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